U.S. patent application number 11/924022 was filed with the patent office on 2008-05-01 for steer drive for tracked vehicles.
This patent application is currently assigned to TORVEC, INC.. Invention is credited to Donald Gabel, James Y. Gleasman, Keith E. Gleasman, Matthew R. Wrona.
Application Number | 20080103005 11/924022 |
Document ID | / |
Family ID | 39345010 |
Filed Date | 2008-05-01 |
United States Patent
Application |
20080103005 |
Kind Code |
A1 |
Gleasman; Keith E. ; et
al. |
May 1, 2008 |
Steer Drive for Tracked Vehicles
Abstract
The differential steering drive for a tracked vehicle includes a
drive differential interconnecting the drive shafts for the tracks
and a steering differential for superimposing additive and
subtractive rotations to the tracks for steering and pivot turning.
In the preferred embodiment for high speed tracked vehicles, the
drive differential is an all-gear no-clutch type limited-slip
differential, and the steering differential is an unlimited-slip
differential. The differentials are arranged to provide no-slip
track operation traveling in straight paths or when steering so
long as at least one track has traction. In another embodiment for
pivot-turning slow-moving off-road vehicles, both the drive
differential and the steering differential are all-gear no-clutch
type limited-slip differentials. Further, both embodiments
preferably include an additional left-side and an additional
right-side all-gear no-clutch type limited-slip differential for
dividing the torque delivered to a respective pair of drive axles
associated with each track.
Inventors: |
Gleasman; Keith E.;
(Fairport, NY) ; Gleasman; James Y.; (Rochester,
NY) ; Gabel; Donald; (Rochester, NY) ; Wrona;
Matthew R.; (Fairport, NY) |
Correspondence
Address: |
MORTON A. POLSTER;BROWN & MICHAELS, P.C.
400 M & T BANK BUILDING
118 N. TIOGA STREET
ITHACA
NY
14850
US
|
Assignee: |
TORVEC, INC.
1999 Mount Read Blvd. Building 3
Rochester
NY
14615
|
Family ID: |
39345010 |
Appl. No.: |
11/924022 |
Filed: |
October 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11553592 |
Oct 27, 2006 |
|
|
|
11924022 |
Oct 25, 2007 |
|
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Current U.S.
Class: |
475/28 |
Current CPC
Class: |
F16H 48/285 20130101;
B62D 11/18 20130101; B62D 11/10 20130101; F16H 48/29 20130101 |
Class at
Publication: |
475/028 |
International
Class: |
B62D 11/10 20060101
B62D011/10 |
Claims
1. A differential steering-drive for a vehicle having respective
left and right driving traction elements, a propulsion engine with
an engine drive shaft, and a steering wheel rotatable by an
operator to indicate an intended direction of travel, said
steering-drive comprising: a drive differential interconnecting
said engine drive shaft and a pair of respective track drive shafts
for differentially driving said respective left and right driving
traction elements; and a steering differential operatively
interconnecting said steering wheel and said respective track drive
shafts so that: rotation of said steering wheel in a first
direction causes rotation of said steering differential in a first
direction and rotation of said steering wheel in an opposite
direction causes rotation of said steering differential in an
opposite direction, the speed of rotation of said steering
differential in each direction being proportional to the angular
rotation of said steering wheel; and the rotation of said steering
differential in a first direction results in the rotation of said
respective track drive shafts in opposite directions; and wherein
said differential steering-drive further comprises one of the
following combinations: (a) at least one of said drive and steering
differentials comprises an all-gear limited-slip differential; and
(b) said drive differential and said steering differential each
comprise an all-gear limited-slip differential.
2. The differential steering-drive of claim 1, wherein each said
respective left and right driving traction element further
comprises a respective plurality of wheels operatively connected to
at least one pair of drive axles; a left-side differential and a
right-side differential each delivering, respectively, divided
torque to each drive axle of one of said respective pairs of drive
axles; said left-side and right-side differentials each being
driven respectively by said pair of respective drive shafts for
differentially driving said respective left and right driving
traction elements; and wherein said left-side and right-side
differentials also each comprise an all-gear limited-slip
differential.
3. The differential steering-drive of claim 2, wherein said
respective left and right driving traction elements each comprise
an endless track in driving contact with said respective plurality
of wheels.
4. The differential steering-drive of claim 3, wherein each said
respective plurality of wheels comprises at least one pair of
tandem wheels in contact with said endless track and wherein one of
said respective drive axles is positioned intermediate said pair of
tandem wheels.
5. The differential steering-drive of claim 1, wherein said
all-gear limited-slip differential comprises a gear complex
comprising: a pair of side-gear worms, each side-gear worm being
mounted for rotation about an output axis and fixed to a respective
output axle; and at least two sets of paired combination gears,
each combination gear of each pair having (a) an axis of rotation
that is substantially perpendicular to said output axis, and (b) a
first gear portion spaced apart from a worm-wheel portion, said
first gear portions of said combination gear pair being in mating
engagement with each other, and said worm-wheel portions of said
combination gear pair being in mating engagement, respectively,
with a respective one of said side-gear worms.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part application of patent
application Ser. No. 11/553,592, filed Oct. 27, 2006, entitled
"STEER DRIVE FOR TRACKED VEHICLES". The aforementioned application
is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention pertains to the field of tracked vehicles.
More particularly, the invention pertains to a steer drive with a
differential for improved performance of a tracked vehicle under
extreme low traction conditions.
[0004] 2. Description of Related Art
[0005] Differential steering systems for tracked vehicles are well
known. Such prior art track steering systems are often identified
by such terms as "double differentials", "steer drives", and
"cross-drive transmissions", and these prior art steering systems
are equally applicable to multi-wheeled off-road vehicles having no
angularly adjustable turning axle. Of this prior art, the Gleasman
steer drive disclosed in U.S. Pat. No. 4,776,235 has proven to be
relatively inexpensive and remarkably effective in testing
conducted on a full-terrain tracked vehicle ("FTV.RTM.") built by
Torvec, Inc. Using the Gleasman steer drive, the operator readily
steers the FTV vehicle with a conventional steering wheel, as
contrasted to the more conventional bulldozer-type drives with
separate left and right control levers for each track, when
traversing paved highways at highway speeds as well as when
traversing off-road terrain.
[0006] Teachings of the prior art indicate that only some
conventional form of unlimited-slip differential gearing may be
used between the vehicle's engine and the track drives so as not to
impair differential rotation of the drive axle shafts. All prior
art differential steering drives for tracked vehicles use some
conventional form of unlimited-slip differential gearing between
the vehicle's engine and the track drives. Apparently, persons
skilled in the art have believed that such a drive differential
must be a differential lacking any limited-slip devices.
[0007] During extensive testing, a problem has been noticed when
the FTV tracked vehicle is being turned on terrain that includes
portions having unusually low traction. For instance, where one
track of the vehicle is traversing extremely soft mud, that track
can occasionally lose all traction and begin to "slip". This is
similar to the undesirable slipping that occurs in a truck with a
conventional unlimited-slip differential, where one set of drive
wheels begins to slip on mud, ice, or snow. When the FTV vehicle is
turning and the entire track on one side of the vehicle loses
traction, the turn is interrupted. In other types of differential
drives if the track continues to slip when turning, the drive
torque of the vehicle can be completely lost.
[0008] As explained in U.S. Pat. No. 4,776,235, the Gleasman steer
drive is "no-slip" so long as the tracked vehicle is moving
straight ahead or straight back and the steering wheel is held
still by the operator. This no-slip condition results from the fact
that the drives of both tracks are locked together when the
steering worm/worm-wheel combination of the vehicle's steer drive
is held motionless. Under this condition, the track drive shafts
operate as if they were on straight axles without any separating
differential. Nonetheless, when the steering motor drive of this
prior art steer drive superimposes different track speeds for
turning, the steering worm/worm-wheel combination begins to rotate,
and this locked condition is lost. That is, the steer drive
introduces differential action between the tracks, and when the
drive shafts are differentiating, the loss of drive torque, i.e.,
slipping, may occur as it does in all conventional unlimited-slip
differentials when one drive axle loses traction.
[0009] The sharpest turn that a conventional bulldozer-type drive,
with separate left and right control levers for each track, can
make is by braking one track while driving the other track, and
this stresses the braked track considerably. Pivot turns using the
Gleasman steer drive involve changing the direction of the vehicle
with little or no translational movement of the pivot point at the
center of the vehicle. Pivot turns can be power-assisted or powered
totally by driving torque to be executed more rapidly. Since a
vehicle is not using driving torque for forward or rearward
movement when pivot turning occurs, driving torque is available for
powering pivot turns. A slippage, similar to the turn slippage
described previously, occurs during pivot turning, when one of the
tracks is mired in a low-traction medium.
[0010] The interruption of steering or the loss of drive torque
when one track slips, is endemic in all differential track drives
and has apparently occurred in steer-driven tracked vehicles since
their inception. As indicated in documentary information provided
on television for the public with the consent of the United States
government, this same slipping condition occurs with steer driven
U.S. Army Abrams tanks. Abrams tanks also include a steering-wheel
type drive in contrast to the more conventional bulldozer-type
drives with separate left and right control levers for each track.
While this condition is not sufficient to detract from the many
advantages of tracked vehicles, it certainly has been a problem
that has been plaguing tracked vehicles for a long time, and it
occurs often enough in severe off-road terrain to justify
correction. Avoidance of such undesirable steering problems is of
particular importance for those few tracked vehicles that are
capable of traveling at highway speeds.
[0011] There is a need in the art for a steer drive that prevents
slippage when torque is suddenly reduced and that facilitates pivot
turning for the tracked vehicle under extreme low traction
conditions.
SUMMARY OF THE INVENTION
[0012] The differential steering drive for a tracked vehicle
includes a drive differential interconnecting the respective drive
shafts for the tracks and a steering differential for superimposing
respective additive and subtractive rotations to the tracks for
steering and pivot turning. In a preferred embodiment for high
speed tracked vehicles, the drive differential is an all-gear
no-clutch type limited-slip differential, and the steering
differential is an unlimited-slip differential. The two
differentials are arranged to provide no-slip track operation
traveling in straight paths or when steering under all conditions
so long as at least one track has traction. In another embodiment,
both the drive differential and the steering differential are
all-gear no-clutch type limited-slip differentials. This second
embodiment may be appropriate for pivot turning some slower moving
off-road vehicles.
[0013] The differential steering-drive for a vehicle includes a
drive differential and a steering differential. The vehicle
includes respective left and right driving tracks or driving
traction elements, a propulsion engine with an engine drive shaft,
and a steering wheel rotatable by an operator to indicate an
intended direction of travel.
[0014] The drive differential interconnects the engine drive shaft
and a pair of respective drive shafts for differentially driving
the respective left and right driving traction elements. The
steering differential operatively interconnects the steering wheel
and the respective track drive shafts so that rotation of the
steering wheel in a first direction causes rotation of the steering
differential in a first direction and rotation of the steering
wheel in the opposite direction causes rotation of the steering
differential in an opposite direction. The speed of rotation of the
steering differential in each direction is proportional to the
angular rotation of the steering wheel. The rotation of the
steering differential in a first direction results in the rotation
of the respective track drive shafts in opposite directions. In one
embodiment, at least one of the drive and steering differentials
includes an all-gear limited-slip differential.
[0015] In the preferred embodiment, the drive differential includes
an all-gear limited-slip differential. In a second embodiment, the
drive differential includes an all-gear limited-slip differential
and the steering differential includes an all-gear limited-slip
differential.
[0016] Both embodiments are also extended to provide an additional
left-side all-gear limited-slip differential and an additional
right-side all-gear limited-slip differential for dividing the
torque delivered to a respective pair of drive axles associated
with each track. That is, while the first two all-gear limited-slip
differentials divide the torque between the respective drive shafts
directing the engine torque to the respective left and right
tracks, the two additional all-gear limited-slip differentials
further divide each respective track torque between the front and
rear drive axles of each respective track.
[0017] The all-gear limited-slip differential preferably includes a
crossed-axis gear complex having a pair of side-gear worms and at
least two sets of paired combination gears. Each side-gear worm is
mounted for rotation about an output axis and fixed to a respective
output axle. Each combination gear has an axis of rotation that is
substantially perpendicular to the output axis. Each combination
gear also has a first gear portion spaced apart from a worm-wheel
portion. The first gear portions of the combination gear pair are
in mating engagement with each other, and the worm-wheel portions
of the combination gear pair are in mating engagement with a
respective one of the side-gear worms. The all-gear limited-slip
differential preferably includes a thrust plate maintained in a
fixed position between the inner ends of the pair of side-gear
worms.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows a partially cross-sectioned side view of a
first full-traction differential for use in the present
invention.
[0019] FIG. 2A is a schematic cross section of a second
full-traction differential for use in the present invention
including a complete worm/worm-wheel gear complex incorporated
within a one-piece housing.
[0020] FIG. 2B is a schematic cross section as viewed along line
2B-2B of FIG. 2A.
[0021] FIG. 3 is a partially schematic view of a steer drive
according to the present invention.
[0022] FIG. 4 shows a schematic view of a tracked vehicle executing
a pivot turn made possible by the present invention.
[0023] FIG. 5 is a schematic view of a preferred embodiment of the
present invention used in a tracked vehicle.
[0024] FIG. 6 is an enlarged schematic partially cross sectional
top view, with some parts and cross-hatching omitted to enhance
clarity, of selected portions of the drive and steering
differentials as well as the left-side and right-side differentials
shown in FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention is related to the subject matter of
U.S. Pat. No. 3,735,647, "SYNCLINAL GEARING", issued to Gleasman on
May 29, 1973; U.S. Pat. No. 4,776,235, "NO-SLIP, IMPOSED
DIFFERENTIAL REDUCTION DRIVE", issued to Gleasman et al. on Oct.
11, 1988; U.S. Pat. No. 6,135,220, "MODULAR SYSTEM FOR TRACK-LAYING
VEHICLE", issued to Gleasman et al. on Oct. 24, 2000; and U.S. Pat.
No. 6,783,476, "COMPACT FULL-TRACTION DIFFERENTIAL", issued to
Gleasman et al. on Aug. 31, 2004, all of which are hereby
incorporated by reference herein.
[0026] Teachings of the prior art steer-drives indicate that only
conventional forms of unlimited-slip differential gearing may be
used between a vehicle's engine and the track drives so as not to
impair differential rotation of the drive axle shafts. However,
undesirable slipping often occurs in a tracked vehicle when the
vehicle is being steered, because the steering drive motor is
moving the otherwise locked-up drive of the steering differential
and, thus, both differentials are differentiating. Under this
condition, should one of the tracks suddenly lose traction, the
torque load becomes significantly out of balance, allowing the
slipping track to increase in speed and reducing the speed and
drive torque on the other track in relation to the increased speed
of the slipping track. [NOTE: Persons skilled in the art will
appreciate that, although the traction elements referred to herein
are primarily "tracks", the multiple-wheel units used to support
and drive the tracks can, and have been, used by themselves as
vehicle traction elements and, therefore, the disclosed
differential steering-drive of the invention can also be
appropriately used to control the drive shafts of such respective
left and right multi-wheel traction elements for steering such a
multi-wheel vehicle.]
[0027] At least one of the drive and steering differentials of the
present invention is an all-gear limited-slip type of differential
as opposed to the conventional unlimited-slip differentials taught
in the prior art. A limited-slip differential allows for a
difference in rotational velocities of the differentiating output
shafts but does not allow the difference to increase beyond a set
amount. Some all-gear differentials cause the gears to bind
together or against the housing to provide a torque bias when
traction is lost. However, the preferred all-gear limited-slip
differentials of the invention use the mechanical advantage of the
worm-like design of the side gears operating against the worm-wheel
design of the combination gears to allow normal differential action
around a turn, and should the traction under one drive component
become significantly less than the traction under the other drive
component, this same mechanical advantage prevents the transfer of
excess torque to the drive component with less traction.
Increasingly greater torque is transferred to the traction
component having greater traction until the difference in the
torque being transferred to each drive component reaches a
predetermined torque bias ratio. The gear design determines the
torque bias ratio, which is the ratio of torque applied to the
traction component with better traction to the torque applied to
the component having lesser traction.
[0028] In the preferred embodiment, the drive differential is an
all-gear limited-slip type differential, and the steering
differential is a conventional unlimited-slip differential. In a
second embodiment, the drive differential and the steering
differential are both all-gear limited-slip type differentials. A
further embodiment extends each of these just-identified
embodiments by combining them with additional right- and left-side
differentials, both of which are all-gear limited-slip type
differentials, for distributing torque to the front and rear
drive-axles for each of the vehicle's respective left and right
driving traction elements.
[0029] The use of an all-gear limited-slip type of differential as
the drive differential of the steer drive prevents the
above-described condition that occurs when traction is suddenly
reduced under one drive member. While any all-gear limited-slip
differential may be used in any steer drive of the present
invention, the all-gear differentials discussed herein are
preferred, namely, the older crossed-axis design shown in FIG. 1
that was widely used under the trademark "Torsen.RTM." or the more
recent compact crossed-axis design shown in FIGS. 2A and 2B and
identified commercially by the trademark "IsoTorque.TM.". As just
stated above, avoidance of such undesirable steering problems is of
particular importance for those few tracked vehicles that are
capable of traveling at highway speeds. This important revision,
however, does not otherwise affect the operation of the basic
features of the original steer-drive, which continues to function
in the same manner. Namely, when the vehicle is being driven in a
straight direction, the differentials still both act as straight
axles, and when the vehicle operator indicates a change in
direction by turning the vehicle's steering wheel, the steering
motor still turns the housing of the steering differential either
forward or in reverse, and the speeds of the tracks are
respectively increased and decreased to accomplish the change of
direction as explained in U.S. Pat. No. 4,776,235.
[0030] With the present invention's use of the limited-slip
differential, pivot turns still change the direction of the vehicle
with little or no translational movement of the pivot point at the
center of the vehicle. Pivot turns are still preferably powered
totally by substantial torque provided by the separate differential
steering system motor, since the torque of that steering motor is
still greatly increased by the worm/worm-wheel gearing ratio
(preferably >15:1).
[0031] During such pivot turning with prior steering systems, the
vehicle operator generally applies a brake to, or otherwise holds,
the engine drive shaft in a locked condition. However, when pivot
turning with heavy, relatively slow-moving off-road vehicles,
conditions arise such that it is not desirable to lock the engine
drive shaft. In these latter instances, should the traction load
being shared between the tracks become significantly unbalanced,
the pivoting motion may be completely stopped. This pivot turning
problem in prior steering systems is avoided in the present
invention by replacing the traditional steering differential with
an all-gear limited-slip type of differential that does not slip
when such torque imbalance occurs. Nonetheless, for all
faster-moving track-laying vehicles, the steering differential
should preferably remain a conventional unlimited-slip all-gear
type.
Limited-Slip Differential
[0032] As shown in FIG. 1, a first embodiment of a limited-slip
differential for use in the present invention includes a rotatable
gear housing 10 and a pair of drive axles 11, 12 received in bores
formed in the sides of housing 10. This type of differential, as
disclosed in U.S. Pat. No. 3,735,647, has enjoyed fairly widespread
use and publicity throughout the world under the Torsen.RTM.
trademark. This limited-slip differential, which is an all-gear
differential, includes no slipping plates or other form of clutch
apparatus and uses either a crossed-axis or a parallel axis
arrangement in a "compound planetary gear complex" format. While
either of these formats may be used, the crossed-axis differential
format is preferred, and only this format is explained with greater
particularity in the following discussion.
[0033] A flange 13 is preferably formed at one end of housing 10
for mounting a ring gear (not shown) for providing rotational power
from an external power source, typically from a vehicle's engine.
The gear arrangement within housing 10 is often called a
"crossed-axis compound planetary gear complex" and preferably
includes a pair of side-gear worms 14, fixed, respectively, to the
inner ends of axles 11, 12 and several sets of combination gears 16
organized in pairs. Each combination gear preferably has outer ends
formed with integral spur gear portions 17 spaced apart from a
"worm-wheel" portion 18. While standard gear nomenclature uses the
term "worm-gear" to describe the mate to a "worm", this often
becomes confusing when describing the various gearing of an
all-gear differential. Therefore, as used herein, the mate to a
worm is called a "worm-wheel".
[0034] Each pair of combination gears 16 is preferably mounted
within slots or bores formed in the main body of housing 10 so that
each combination gear rotates on an axis that is substantially
perpendicular to the axis of rotation of side-gear worms 14, 15. In
order to facilitate assembly, each combination gear 16 preferably
has a full-length axial hole through which a respective mounting
shaft 19 is received for rotational support within journals formed
in housing 10.
[0035] Combination gears are known with integral hubs that are
received into the journals of housing 10, but to facilitate design
of the housing and assembly, the combination gears of most
presently-used limited-slip differentials of this type are
shaft-mounted. The spur gear portions 17 of the combination gears
16 of each pair are in mesh with each other, while the worm-wheel
portions 18 are, respectively, in mesh with one of the side-gear
worms 14, 15 for transferring and dividing torque between axle ends
11, 12. In order to carry most automotive loads, prior art
differentials of this type usually include three sets of paired
combination gears positioned at approximately 120.degree. intervals
about the periphery of each side-gear worm 14, 15.
[0036] This type of differential does a remarkable job of
preventing undesirable wheel slip under most conditions. In fact,
one or more of these limited-slip differentials are either standard
or optional on vehicles presently being sold by at least eight
major automobile companies throughout the world, and there are two
Torsen.RTM. crossed-axis limited-slip differentials in every U.S.
Army HMMWV ("Hummer") vehicle, one differentiating between the
front wheels and the other between the rear wheels.
[0037] A second embodiment of a more recent design of limited-slip
differential for use in the present invention is shown in FIGS. 2A
and 2B. This second embodiment is disclosed in greater detail in
copending application Ser. No. 11/553,603, filed Oct. 27, 2006 and
entitled "Full-Traction Differential with Hybrid Gearing", and is
presently being marketed under the name "IsoTorque.TM.". The
contact pattern of this new design spreads the load over such a
significantly wider area that it is possible to use only two pairs
of combination gears (spaced, respectively, at 180.degree.
intervals) rather than the more conventional three pairs (spaced,
respectively, at 120.degree. intervals) to carry a given load. That
is, this improved tooth design creates greater areas of tooth
engagement as well as increasing the number of teeth in contact at
any given time, making it possible to meet automotive
specifications with two fewer gears. Of course, this same tooth
design can make it possible to carry significantly greater loads
with the conventional three pairs of combination gears. Also, as
different from conventional line contact that concentrates the
load, the contact pattern of this gearing spreads the load over a
relatively larger area and results in less shearing of the
lubricating oil film, thereby permitting the use of lower viscosity
lubricants and assuring longer part life.
[0038] A salient feature of the crossed-axis gear complex of
high-traction differentials is the mechanical advantage resulting
from the worm/worm-wheel combination in the gear train between the
vehicle's wheels and the differential. As a vehicle travels around
curves, the weight and inertia of the vehicle cause the wheels to
roll simultaneously over the surface of the road at varying speeds,
resulting in the need for differentiation. The initiation of such
differentiation is greatly enhanced by a mechanical advantage
between the side-gear worms and their mating worm-wheels. Of
course, this same gearing results in mechanical disadvantage when
torque is being transferred in the opposite direction. The
preferred embodiments of the IsoTorque.TM. differential select
35.degree./55.degree. for the worm/worm-wheel teeth to provide both
full traction as well as relative ease of differentiation, a
selection that also makes the differential particularly appropriate
for vehicles including automatic braking systems (ABS) having
traction controls.
[0039] A further feature of the IsoTorque.TM. differential provides
torque balancing that equalizes the end thrust on the respective
side-gear worms during vehicle turning, when being driven in either
forward or reverse, regardless of the direction of travel. A thrust
plate is supported by the same mounting that supports the sets of
paired combination gears, the thrust plate being fixed against
lateral movement and maintained between the inner ends of the
side-gear worms. Thus, when under thrust to the left, the right
worm exerts a thrust force x against the thrust plate, and the left
worm exerts only its own thrust force x against the housing rather
than the 2x force as in previous differentials. Similarly, when
under thrust to the right, the left worm exerts a thrust force x
against the thrust plate, and the right worm exerts only its own
thrust force x against the housing.
[0040] This just described torque-balancing feature can be seen in
the second embodiment shown in FIGS. 2A and 2B. The differential
incorporates a complete worm/worm-wheel gear complex. The housing
120 is formed, preferably, in one piece from powder metal and has
only three openings. Namely, a first set of appropriate openings
121, 122 is aligned along a first axis 125 for receiving the
respective inner ends of output axles (not shown), and only a
single further opening 126, which is rectangular in shape and
extends directly through housing 120, is centered perpendicular to
axis 125.
[0041] Two pair of combination gears 131, 132 and 129, 130 each
have respective spur gear portions 133 separated by an
hourglass-shaped worm-wheel portion 134 that are designed and
manufactured as described above. The respective spur gear portions
133 of each pair are in mesh with each other, and all of these
combination gears are rotatably supported on sets of paired hubs
136, 137 that are formed integrally with an opposing pair of
mounting plates 138, 139. The respective worm-wheel portions 134 of
combination gear pair 131, 132 are in mesh with respective ones of
a pair of side-gear worms 141, 142, while the respective worm-wheel
portions 134 of combination gear pair 129, 130 are similarly in
mesh with, respectively, the same pair of side-gear worms 141,
142.
[0042] Positioned intermediate the inner ends of side-gear worms
141, 142 is a thrust plate 150. Thrust plate 150 includes
respective bearing surfaces 152, 153, mounting tabs 156, 157, and a
weight-saving lubrication opening (not shown). Mounting tabs 156,
157 are designed to mate with slots 160, 161 formed centrally in
identical mounting plates 138, 139. Slots 160, 161 not only
position thrust plate 150 intermediate the inner ends of side-gear
worms 141, 142 but also prevent lateral movement of thrust plate
150. Therefore, referring specifically to FIG. 2A, when driving
torque applied to side-gear worms 141, 142 results in thrust to the
left, worm 142 moves against fixed bearing surface 152 of thrust
plate 150, while worm 141 moves away from fixed bearing surface 153
of thrust plate 150 and against housing 120 (or against appropriate
washers positioned conventionally between worm 141 and housing
120). The resulting friction against the rotation of worm 141 is
unaffected by the thrust forces acting on worm 142.
[0043] Similarly, when driving torque applied to side-gear worms
141, 142 results in thrust to the right, worm 141 moves against
fixed bearing surface 153 of thrust plate 150, while worm, 142
moves away from fixed bearing surface 152 of thrust plate 150 and
against housing 120 (or, again, against appropriate washers
positioned conventionally between worm 142 and housing 120).
Similarly, the resulting friction against the rotation of worm 142
is unaffected by the thrust forces acting on worm 141. Thus,
regardless of the direction of the driving torque, the friction
acting against the rotation of each side-gear worm is not affected
by the thrust forces acting on the other side-gear worm. Since the
torque bias of the differential is affected by frictional forces,
this prevention of additive thrust forces helps to minimize torque
imbalance, i.e., differences in torque during different directions
of vehicle turning.
Steer Drive Structure
[0044] As shown in FIG. 3, when a steer drive of the present
invention 20 is applied to a vehicle, engine power input via shaft
21 turning gear 22 rotates ring gear 23 and case 24 of a drive
differential 25. Drive differential 25 is connected for driving a
pair of respective axle shafts 26 and 27 for differentially driving
respective left and right driving traction elements on opposite
sides of the vehicle. Drive differential 25 is suitably sized to
the vehicle being driven. This can range from small garden tractors
and tillers up to large tractors and earth movers.
[0045] A steering differential 30 having a case 29 is connected
between a pair of steering control shafts 32 and 33 that are
interconnected in a driving relationship with axle drive shafts 26
and 27. One steering control shaft 33 and one axle drive shaft 27
are connected for rotation in the same direction, and another
steering control shaft 32 and another axle drive shaft 26 are
connected for rotation in opposite directions. This causes counter
or differential rotation of control shafts 32 and 33 as axle shafts
26 and 27 rotate in the same direction and conversely causes
differential rotation of axle shafts 26 and 27 as control shafts 32
and 33 rotate in the same direction.
[0046] At least one of the differentials 25, 30 of the present
invention is an all-gear limited-slip type of differential (e.g.,
the "Synclinal Gearing" disclosed in U.S. Pat. No. 3,735,647, the
"Compact Full-Traction Differential" disclosed in U.S. Pat. No.
6,783,476, or the "Full-Traction Differential" disclosed in
copending application Ser. No. 11/553,603, filed Oct. 27, 2006).
This is in opposition to the teachings of the prior art that
clearly teach using only unlimited-slip differentials. In the
preferred embodiment of the invention, the drive differential 25 is
an all-gear limited-slip type differential, and the steering
differential 30 is a conventional unlimited-slip differential. In
another embodiment of the invention, the drive differential 25 is a
conventional unlimited-slip differential, and the steering
differential 30 is an all-gear limited-slip type differential.
[0047] As shown in FIG. 3, gear connections between steering
control shafts and axle drive shafts are preferred for larger and
more powerful vehicles. These include axle shaft gears 35 and 36
fixed respectively to axle shafts 26 and 27 and control shaft gears
37 and 38 fixed respectively to control shafts 32 and 33. Meshing
axle shaft gear 35 with control shaft gear 37 provides opposite
rotation between axle shaft 26 and control shaft 32, and meshing
both axle shaft gear 36 and control shaft gear 38 with idler gear
34 provides same direction rotation for axle shaft 27 and control
shaft 33.
[0048] Gear connections between steering control shafts and axle
drive shafts are preferably incorporated into an enlarged housing
containing both drive differential 25 and steering differential 30.
For a reason explained below, steering differential 30 can be sized
to bear half the force borne by drive differential 25 so that the
complete assembly can be fitted within a differential housing that
is not unduly large.
[0049] Smaller or less powerful vehicles can use shaft
interconnections such as belts or chains in place of gearing. Also,
shaft interconnections need not be limited to the region of the
axle differential and can be made toward the outer ends of the axle
shafts.
[0050] A gear or drive ratio between steering control shafts and
axle drive shafts is preferably 1:1. This ratio can vary, however,
so long as it is the same on opposite sides of the axle and control
differentials.
[0051] An input steering gear 40 meshes with a ring gear 31 fixed
to casing 29 of steering differential 30 for imposing differential
rotation on the system. Gear 40 is preferably a worm gear, and ring
gear 31 is preferably a worm-wheel so that ring gear 31 turns only
when gear 40 turns.
[0052] For steering purposes, steering gear 40 can be turned by
several mechanisms, depending on the relative loads. Steering
mechanisms can use various types of appropriately sized motors for
turning gear 40. For instance, a DC starter motor 41 can be
electrically energized via a rheostat in the steering system, or a
hydraulic or pneumatic motor 41 can be turned by a vehicle's
hydraulic or pneumatic system in response to a steering control.
Preferably, motor 41 is hydraulic, and the worm 40/worm-wheel 31
ratio is at least 15:1.
[0053] As indicated above, slipping occurs with prior art
differential steering systems when the vehicle is being steered
because the steering drive motor is moving the otherwise locked-up
worm/worm-wheel drive of the steering differential and, thus, both
differentials are differentiating. Under this condition, should one
of the tracks suddenly lose traction, the torque imbalance allows
the slipping track to increase in speed, reducing the drive torque
and speed of the other track in direct relation to the increased
speed of the slipping track in prior art systems.
[0054] When the conventional differential used by prior art
differential steering systems for drive differential 25 is
replaced, as indicated above in the preferred embodiment of the
present invention, with an all-gear limited-slip differential
(e.g., the IsoTorque.TM. differential described in U.S. Pat. No.
6,783,476) that does not slip when torque is suddenly reduced, this
undesirable condition is prevented.
[0055] However, it is important to note that this revision does not
otherwise affect the operation of the basic steer-drive, which
continues to function in the same manner. Namely, when the vehicle
is being driven in a straight direction, the non-rotation of the
steering gear 40/ring gear 31 combination still causes both
differentials to act as straight axles, and when the vehicle
operator indicates a change in direction by turning the vehicle's
steering wheel, the steering motor turns the housing of the
differential either forward or in reverse, and the speeds of the
tracks are respectively increased and decreased to accomplish the
change of direction as explained in U.S. Pat. No. 4,776,235.
[0056] However, since the invention's drive differential 25 is an
all-gear limited-slip differential, whenever the torque load shared
by the tracks suddenly begins to become unbalanced, the torque bias
of drive differential 25 immediately transfers a substantial
portion of the drive torque received from engine input shaft 21 to
the track having the better traction (e.g., up to eight times as
much torque in a 8:1 differential). Thus, as soon as the traction
load on either track results in a significant load imbalance, a
sufficient portion of the drive torque is still delivered to the
track having better traction to maintain movement of the tracked
vehicle.
No-Slip Steer-Drive Operation and Pivot Turning
[0057] Two important effects occur from the interconnection of
steering differential 30 and its control shafts 32 and 33 with axle
drive differential 25 and axle shafts 26 and 27. One is a no-slip
drive that prevents wheels or tracks from slipping unless slippage
occurs on both sides of the vehicle at once. The other effect is
imposed differential rotation that can accomplish steering to pivot
or turn a vehicle.
[0058] The no-slip drive occurs because axle drive shafts 26 and 27
are geared together via steering differential 30. Power applied to
an axle shaft on a side of the vehicle that has lost traction is
transmitted to the connecting control shaft on that side, through
differential 30 to the opposite control shaft, and back to the
opposite axle shaft where it is added to the side having traction.
So if one axle shaft loses traction, the opposite axle shaft drives
harder, and the only way slippage can occur is if both axle shafts
lose traction simultaneously.
[0059] To elaborate on this, consider a vehicle rolling straight
ahead with its axle shafts 26 and 27 turning uniformly in the same
direction. Steering gear 40 is stationary for straight ahead
motion, and since steering gear 40 is preferably a worm gear,
worm-wheel 31 of steering differential 30 cannot turn. Control
shafts 32 and 33, by their driving connections with the axle drive
shafts, rotate differentially in opposite directions, which
steering differential 30 accommodates.
[0060] Drive differential 25 equally divides the power input from
engine drive shaft 21 and applies half of the input power to each
axle shaft 26 and 27. If the track or wheel being driven by axle
shaft 26 loses traction, it cannot apply the power available on
shaft 26 and tends to slip. Actual slippage cannot occur, however,
because axle shaft 26 is geared to control shaft 32. So if a wheel
or track without traction cannot apply the power on shaft 26, this
is transmitted to control shaft 32, which rotates in an opposite
direction from axle shaft 26. Since ring gear 31 cannot turn,
rotational power on control shaft 32 is transmitted through
differential 30 to produce opposite rotation of control shaft 33.
This is geared to axle shaft 27 via idler gear 34 so that power on
control shaft 33 is applied to axle shaft 27 to urge shaft 27 in a
forward direction, driving the wheel or track that has traction and
can accept the available power. Since only half of the full
available power can be transmitted from one axle shaft to another
via differential 30 and its control shafts, these can be sized to
bear half the force borne by axle differential 25 and its axle
shafts.
[0061] Of course, unusable power available on axle shaft 27,
because of a loss of traction on that side of the vehicle, is
transmitted through the same control shaft and control differential
route to opposite axle shaft 26. This arrangement applies the most
power to the wheel or track having the best traction, which is
ideal for advancing the vehicle. The wheel or track that has lost
traction maintains rolling engagement with the ground while the
other wheel or track drives. The only time wheels or tracks can
slip is when they both lose traction simultaneously.
[0062] To impose differential rotation on axle shafts 26 and 27 for
pivoting or turning the vehicle, it is still only necessary to
rotate steering gear 40. This differentially rotates axle shafts to
turn or pivot the vehicle because of the different distances
traveled by the differentially rotating wheels or tracks on
opposite sides of the vehicle.
[0063] Whenever steering gear 40 turns, it rotates ring gear 31,
which turns the casing 29 of steering differential 30 to rotate
control shafts 32 and 33 in the same direction. The connection of
control shafts 32 and 33 with axle drive shafts 26 and 27 converts
the same direction rotation of control shafts 32 and 33 to opposite
differential rotation of axle shafts 26 and 27, as accommodated by
drive differential 25. This drives wheels or tracks forward on one
side of the vehicle and rearward on the other side of the vehicle,
depending on the direction of rotation of steering gear 40.
[0064] Such differential rotation is added to whatever forward or
rearward rotation of the axle shafts is occurring at the time. So
if a vehicle is moving forward or backward when steering gear 40
turns, the differential rotation advances and retards opposite axle
shafts and makes the vehicle turn.
[0065] If a vehicle is not otherwise moving when steering gear 40
turns, the vehicle's left and right driving traction elements
(wheels or tracks) go forward on one side and backward on the other
side so that the vehicle pivots on a central point. Such a pivot
turn is schematically illustrated in FIG. 4 for a vehicle having a
pair of tracks 85 and 86. Both tracks can have a rolling engagement
with the ground as the vehicle rotates around a center point 87 by
driving right track 86' forward and left track 85' rearward. The
tracks experience some heel and toe scuffing, but this causes less
stress and disturbance of the terrain than is caused by the
traditional locking of one track by a brake while the other track
is driven. The pivot turn also spins the vehicle on one point 87,
without requiring motion in any direction as must occur when one
track is braked and another is driven.
[0066] In prior art steer drives, the above-described no-slip drive
functions only so long as the vehicle is traveling straight ahead
or straight back and steering gear 40 and steering differential 30
are not operating in response to the driver's rotation of the
vehicle's steering wheel. However, as explained above, in prior art
steer drives, when steering differential 30 is differentiating and
one of the tracks completely loses traction, the steer drive
introduces differential action between the tracks, and the drive
torque of the vehicle can still be completely lost if that track
continues to slip. This total loss of driving torque does not occur
with the improved steer drive of the invention herein.
[0067] Namely, since drive differential 25 is an all-gear
limited-slip differential, whenever the torque load shared by the
tracks suddenly begins to become unbalanced, the torque bias of
drive differential 25 immediately transfers a substantial portion
of the drive torque received from engine input shaft 21 to the
track having the better traction (e.g., this transfer of drive
torque occurs up to a torque imbalance of eight times in an 8:1
differential). Thus, as soon as the traction load on either track
results in a significant load imbalance, a sufficient portion of
the drive torque is still delivered to the track having better
traction to maintain movement of the tracked vehicle.
Improved Pivot Turning
[0068] As indicated above, during pivot turning with prior
differential steering systems, the operator of a tracked vehicle
generally applies a brake to, or otherwise holds, the engine drive
shaft in a locked condition. With heavy, relatively slow-moving
off-road vehicles operating in terrain where traction can vary
greatly between tracks, conditions arise when pivot turning is
desired but the usual locking of the engine drive shaft is not
appropriate. As explained above, under such conditions, severe
traction imbalance can result in undesirable loss of pivot turn
motion.
[0069] To facilitate pivot turning for such vehicles, the present
invention replaces the traditional steering differential with an
all-gear limited-slip type of differential (e.g., IsoTorque.TM.
differential), as previously described above, that does not slip
when torque imbalance occurs. This simple change overcomes pivot
turn problems under all conditions so long as one track retains
traction. Namely, in this just-described second embodiment of the
present invention, steering differential 30 is an all-gear
limited-slip type of differential that prevents slip when traction
is suddenly reduced under one track when pivot turning a
slow-moving off-road tracked vehicle.
Both Embodiments Enhanced
[0070] The embodiments just described above can both be enhanced by
providing an additional left-side all-gear limited-slip
differential and an additional right-side all-gear limited-slip
differential for dividing the torque delivered to a respective pair
of drive axles associated with each track. This extension
embodiment is illustrated schematically in FIGS. 5 and 6. FIG. 5 is
a schematic top view of the drive elements of a tracked vehicle,
showing (in darker lines) the invention's all-gear limited-slip
steer drive 218 in combination with two additional all-gear
limited-slip differentials, namely, right-side differential 250 and
left-side differential 251, while FIG. 6 is an enlarged partially
schematic view of these four last-named differentials.
[0071] The drive path for the tracked vehicle (shown in FIGS. 5 and
6) is as follows: an engine 210 is connected to a transmission 212
for transmitting torque to a central drive shaft 214 that drives a
pair of bevel gears 220, 221 delivering driving torque to the
steer-drive unit 218, the bevel gear 221 providing differentiated
driving and steering torque to the vehicle's respective left and
right driving traction elements through respective right drive
shaft 226 and left drive shaft 227, in the manner explained in
considerable detail above.
[0072] Respective drive shafts 226, 227 operate respective bevel
gears 253, 254 and 255, 256 delivering driving torque to right-side
differential 250 and left-side differential 251 that, respectively,
further differentiate their respective driving torque through their
respective front drive shafts 258, 259 and rear drive shafts 260,
261, the drive shafts 258, 259, 260, 261 being connected
respectively to front right-angle boxes 262, 263 and rear
right-angle boxes 264, 265. As is well known in the art, the
right-angle boxes include pairs of bevel gears (not shown) that
deliver respective torque to front and rear pairs of drive axles,
namely, front right- and left-drive axles 266, 267 and rear right-
and left-drive axles 268, 269. Each drive axle is positioned
between a pair of tandem wheels, e.g., front right-drive axle 266
is positioned between tandem pair of wheels 270, 271, driving at
least one wheel of each tandem pair by means of a respective chain
272.
[0073] In the preferred tracked vehicle shown, each wheel is a dual
wheel, and the respective right and left tracks 274, 275 are
positioned over the mating surfaces of the sets of dual wheels
mounted on each side of the vehicle, all in the manner well known
in the art and explained in detail in above-cited U.S. Pat. No.
6,135,220.
[0074] Referring to FIG. 6, steering differential 230 and its
connecting shafts 231, 232 and gears 233, 234, 235, 236, 237, 238
all operate in exactly the same manner as the corresponding parts
that are shown in FIG. 3 and explained in detail above. The
steering gear worm 240 and the motor 241 to turn steering gear worm
240 are also shown in FIG. 6. Motor 241 is preferably either a DC
motor or a hydraulic motor responsive to indications of the desired
direction of vehicle operation generated by the vehicle's steering
wheel.
[0075] Each additional all-gear limited-slip differential 250, 251
(a) prevents "wind up" between the front and rear portions of its
respective track 274, 275 that might otherwise occur when the
supporting wheels of the track move up and down at different times
over uneven terrain and (b) increases the efficiency of the front
and rear track drives by directing more torque to the respective
drive axle which has the best frictional connection to the track at
any given moment.
[0076] Thus, with the just-described "enhanced" version of the
preferred embodiment of the invention, the all-gear limited-slip
drive differential 224 of steer drive 218 divides the torque
between the respective drive shafts 226, 227 directing the engine
torque to the respective right and left tracks, while the two
additional all-gear limited-slip differentials 250, 251 further
divide each respective track torque between the front and rear
drive axles of each respective track.
[0077] Accordingly, it is to be understood that the embodiments of
the invention herein described are merely illustrative of the
application of the principles of the invention. Reference herein to
details of the illustrated embodiments is not intended to limit the
scope of the claims, which themselves recite those features
regarded as essential to the invention.
* * * * *